An exhaust gas treatment system of an internal combustion engine is described in German Published Patent Application No. 101 39 142 in which an SCR catalytic converter (selective catalytic reduction) is used for reducing the NOx emissions, the catalytic converter reducing the nitrogen oxides contained in the exhaust gas to nitrogen using ammonia as the reagent. The ammonia is extracted from a urea-water solution in a hydrolysis catalytic converter which is situated upstream from the SCR catalytic converter. The hydrolysis catalytic converter converts the urea contained in the urea-water solution into ammonia and carbon dioxide.
The urea-water solution is provided in a reagent tank. The concentration of the urea-water solution is obtained from the signals of a pressure sensor and a temperature sensor. The fluid level of the reagent tank is detected by a fluid level sensor (not shown) and may be indicated to an operator of the internal combustion engine. Removal of the at least one exhaust gas component NOx from the exhaust gas may be ensured only if an adequate quantity of the urea-water solution is available in the reagent tank.
An approach is described in German Published Patent Application No. 197 39 848 with which the NOx raw emissions may be calculated from the known performance characteristics of the internal combustion engine, at least approximately. The starting point is a characteristics field spanned by the fuel injection quantity and the speed of the internal combustion engine. Corrections may be additionally provided, e.g., as a function of air ratio lambda.
An exhaust gas treatment system of an internal combustion engine is known from European Published Patent Application No. 1 024 254 in which an SCR catalytic converter is also used for reducing the NOx emissions. Ammonia, which is extracted in the exhaust duct from a urea-water solution, is again provided as the reagent. The reagent rate is set based on the fuel injection quantity and the engine speed, as well as on at least one parameter of the exhaust gas, the exhaust gas temperature, for example.